Positron Emission Tomography (PET) is a widely-used method of functional imaging. During an examination a weak radioactive substance, of which the distribution in the organism is made visible by way of PET, is administered to a person being examined. This enables biochemical and physiological functions of the organism to be mapped. In such cases molecules which are marked with a radio nuclide which emits positrons are used as radiopharmaceuticals. The high-energy photons, which are emitted at an angle of 180° to each other, produced in the body of the person under examination during the annihilation of the positron with an electron are detected by a plurality of detectors arranged in a ring around the person under examination. Only coincident events which have been detected with two opposite detectors are evaluated in each case.
From the registered coincident decay events deductions are made about the spatial distribution of the radio pharmaceutical within the body and a series of image slices are computed. The image can be reconstructed in such cases with a filtered back projection or an iteration method, with the spatial resolution usually lagging behind the resolution of conventional computer tomography (CT) or magnetic resonance tomography (MRT).
On their passage through material the photons produced during the annihilation can be absorbed, with the absorption probability depending on the path length through the material and the corresponding absorption coefficient of the material. Accordingly in PET a correction of the signals in relation to the attenuation by components which are located in the beam path is necessary. In particular such a correction has to be undertaken if a quantitative analysis of the data is to be carried out for example for quantifying accumulations of the marked substance (i.e. the radiopharmaceutical) in areas of the person under examination. In image reconstruction too not taking account of the absorption of the radiation leads to the occurrence of artifacts, since the measured activity distribution without absorption correction does not match the actual distribution. The correction of the attenuation of the radiation requires the knowledge of the location of the attenuating structures which are taken into account during reconstruction of PET image data by means of an attenuation correction map (μ-map).
An attenuation correction map can be determined with a combined PET/CT system. The correction maps can be calculated in such cases from the Hounsfield values of the CT data. This method is made possible by the x-ray radiation of the CT undergoing a similar attenuation on its passage through the person under examination to the high-energy photons during the recording of the PET signals. Furthermore such systems enable the high local resolution of CT to be combined with the functional imaging of PET.
CT devices have the disadvantage however that damaging x-rays are used and that only a low soft tissue contrast can be achieved without contrast media. However a high soft tissue contrast is desirable, especially in functional imaging of the brain.
A high local resolution with simultaneous high soft tissue contrast as well as a functional imaging can be achieved with a combination of PET and magnetic resonance tomography (MRT). Such a system simultaneously enables high resolution images of the brain structure to be delivered and functional activities in the brain to the mapped. MRT allows different types of tissue to be differentiated, while PET makes physiological and biochemical activities visible. However it is problematic to derive coefficients of attenuation for the high-energy photons of the PET imaging from the MRT image data, i.e. to determine the attenuation correction map. Furthermore the recording of MRT image data demands a significantly longer acquisition time than the creation of computer tomographies.
To take into account the attenuation of the emitted photons through the body any deviations of the MRT imaging from the true geometry are also disruptive. In this case areas of the body which, although they lie in the PET beam path, are not mapped or not mapped at the correct position pose a particular problem. In particular because of the high ratio of the attenuation coefficient (μ value) of human tissue to air it is desirable for a correct attenuation correction to determine the spatial transition from air to tissue as exactly as possible.